In the context, that was during the presidential primary campaign. A disengagement target is the people whom you just don't want to talk to. For example, if you're campaigning, and you know there's a street full of hard-core Conservatives, you're just not going to bother knocking on those doors, and that's who the disengagement targets are. It's a list that you can use so that when, for example, you're making a mailing list to send out mailers to people, you can then take out the people who you know are never going to vote for you because they're hard-core Conservatives or, in the case of the primary campaigns, they're extreme Democrats or something like this.
On June 12th, 2018. See this statement in context.